Electro-magnetic interference (EMI) is known as a critical issue in designing a switching power supply, and is typically improved by spread-spectrum approaches that improve EMI by jittering the switching frequency of the power supply. Existing pulse width modulation (PWM) power supply devices, as those disclosed in U.S. Pat. Nos. 5,929,620, 6,249,876 and 7,289,582, mainly accomplish spectrum-spreading by jittering the frequency of the oscillator and in turn jittering the switching frequency of the power supply. A PFM power supply is a variable-frequency system whose switching frequency varies with its load, so is less subject to EMI. Such a PFM power supply, however, when having a consistent load, has its switching frequency held consistent, and thus still suffers from EMI. Nevertheless, it is infeasible in the PFM power supply to jitter the switching frequency by jittering the frequency of an oscillator that is absent.